Add nginx demo doc

docs/nginx_demo.md

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+# Nginx RTMP module demo
+
+We have modified the included checkout of nginx and its RTMP external module to
+be compartmentalized, with the main nginx server in one compartment and the RTMP
+module in another. These modifications include sharing necessary global data,
+handling function pointers and callbacks that cannot be automatically annotated,
+and modifying heap allocations shared between the main and module compartments.
+We have added support to the nginx pool allocator for compartment-private pools
+and shared pools, configurable when creating the pool.
+
+To build:
+
+    cd external/nginx
+    ./reconfigure
+    cd build/nginx
+    make
+
+The reconfigure script should handle most situations, but is really just a
+convenience for demonstration purposes. See the script for more details on how
+we are building nginx.
+
+## Usage
+
+To run the demo, you must first set up temp directories (these paths are
+configured in external/nginx/conf/nginx.conf, which is then copied to the build
+directory):
+
+    mkdir -p /tmp/nginx/stream
+
+You can then run the nginx binary (external/nginx/build/nginx/nginx). After nginx is running, upload a stream with something like the following (replace `INPUT_FILE.mp4`):
+
+    ffmpeg -re -stream_loop -1 -i INPUT_FILE.mp4 -vcodec libx264 -preset:v ultrafast -acodec aac -f flv rtmp://localhost/stream/demo
+
+And view the stream in a video player (VLC works) at
+`rtmp://localhost/stream/demo`. Nginx will also transcode the stream to HLS in a
+temp folder, just to demonstrate additional processing on the input stream.